lagg(4) and failover

Peter Jeremy peterjeremy at
Tue Aug 12 11:24:41 UTC 2008

On 2008-Aug-12 18:55:52 +0800, Eugene Grosbein <eugen at> wrote:
>On Tue, Aug 12, 2008 at 12:37:15PM +0200, Marian Hettwer wrote:
>> I'm using lagg(4) on some of our servers and I'm just wondering how the
>> failover is implemented.

As far as I can tell, not especially well :-(.  It doesn't seem to detect
much short of layer 1 failure.  In particular, shutting down the switch
port will not trigger a failover.

>> The manpage isn't quite clear:
>>      failover     Sends and receives traffic only through the master port. 
>> If
>>                   the master port becomes unavailable, the next active port
>> is
>>                   used.  The first interface added is the master port; any
>>                   interfaces added after that are used as failover devices.
>> What is meant by "becomes unavailable"? Is it just the physical link which
>> needs to become unavailable to trigger a failover?

It seems to be,

>Yes. It seems you need lacp protocol described later in the manual.

Actually, lacp and failover are used differently: lacp is primarily
used to increase the bandwidth between the host and the switch whilst
failover is used for redundancy.

With lacp, all the physical interfaces must be connected to a single
switch.  With failover, the physical interfaces will normally be
connected to different switches (so a failure in one switch will not
cause the loss of all connectivity.

Peter Jeremy
Please excuse any delays as the result of my ISP's inability to implement
an MTA that is either RFC2821-compliant or matches their claimed behaviour.
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